1. Field of the Invention
The present invention relates to an optical information recording medium and the like, such as an optical disk.
2. Related Art of the Invention
In recent years, optical disks, optical cards and optical tapes have been proposed and developed as media for optically recording information. Above all, optical disks are attracting note as media capable of recording and reproducing information in large capacity and in high density.
One of the erasable optical disk types is a phase change type optical disk. The recording film for use in the phase change type optical disk takes on either an amorphous state or a crystalline state depending on the conditions of heating or cooling with a laser beam, and the two states are reversible between each other. The optical constant of the recording film is different between the amorphous state and the crystalline state. In the phase change type optical disk, one or the other or the two states is selectively formed in the recording film according to information signals, and the resultant optical change (i.e. the change in transmittance or reflectivity) is utilized for recording or reproducing the information signals. In order to achieve the two states, the information signals are recorded by the following method.
When the recording film of the optical disk is irradiated in a pulse form with power for raising the temperature of the recording film above its melting point (hereinafter referred as peak power), the molten portion is quickly cooled with the passage of the laser beam to present a record mark of the amorphous state. Or when the recording film is irradiated with a converted laser beam of an intensity to raise the temperature of the recording film above its crystallization temperature but not over the melting point (hereinafter referred as bias power), the irradiated portion of the recording film turns into the crystalline state.
In addition, a keen requirement has emerged in recent years for higher density of optical disks. In connection with that, there has been proposed a multi-layered recording medium having two or more layers in the thickness direction of the disk wherein information can be recorded onto or reproduced out of each information layer.
However, when recording is to be done onto an information layer farther inside from the laser incidence side by the conventional method, there is a fear that the farther inside information layer is affected by the nearer information layer.
For instance, depending on whether or not any signal is recorded in a recording/reproducing area of an information recording layer, the transmittance of that layer differs. Accordingly there is a problem that, when recording is to be done on a farther information layer, the intensity of the laser beam reaching the farther information layer varies with the ratio between the amorphous area and the crystalline area in the laser spot on the nearer information recording layer, making it impossible to perform accurate recording.
A further problem is that, when data on the farther information layer are to be reproduced, the change in the transmittance depending on the recording state of the nearer information layer invites deterioration in the quality of reproduced signals.